1. Field of the Invention
The present invention relates generally to the transfer of microfluidic quantities of fluids and, in particular, to a tip design and random access tip array for genomic applications and high throughput screening.
2. Background of the Related Art
There is an ongoing effort, both public and private, to spell out the entire human genetic code by determining the structure of all 100,000 or so human genes. Also, simultaneously, there is a venture to use this genetic information for a wide variety of genomic applications. These include, for example, the creation of microarrays of DNA material on targets or substrates to create an array of spots on microscope slides or biochip devices. These arrays can be used to read a particular human's genetic blueprint. The arrays decode the genetic differences that make one person chubbier, happier or more likely to get heart disease than another. Such arrays could detect mutations, or changes in an individual's chemical or genetic make-up, that might reveal something about a disease or a treatment strategy.
It can be a difficult task to efficiently and accurately create DNA microarrays. The desired density of the microarrays can be as high as several thousand dots/cm2. Moreover, the desired volume transfer can be low enough to be in the picoliter range.
One typical way of forming DNA microarrays utilizes pins that can be dipped into solutions of the sample fluid(s) and then touched to a surface to create a small spot or dot. The pins are typically thin rods of stainless steel which have a sharpened fine point to provide a small spot size. Undesirably, the sharp point makes the pins fragile and repeated contact with the surface can lead to damaged pins. This can affect the accuracy of the volume transferred, and hence result in unrepeatable and inconsistent performance. Also, these pins generally allow only a single spot to be formed from a single dip.
More recently, pins have been made with a small slot to permit multiple spotting from a single dip of the sample fluid. Undesirably, the slot can render the pins even more fragile. Another disadvantage of the slotted pin technology is that there is a large variation in the spot size and volume transfer between the first transfer and subsequent transfers—this variation can be as much as 50%. Also, the fluid sample in the slot is undesirably exposed to the atmosphere during the transfer step. This can lead to contamination and evaporation of valuable fluid. Moreover, the pins can have limited reproducibility due to surface tension changes within the slot as solution is dispensed and as solution evaporates from the exposed pin. Additionally, thorough cleaning of the slotted pins can be difficult and time-consuming.
In many cases, the spotting pins are held in a pin holder which allows multiple pins to be dipped into the sample solution and spotted onto the target, typically a glass slide. The spacing between the pins typically corresponds to the spacing between the wells of the source plate. To create high density microarrays, the pins are simultaneously dipped and then spotted. Subsequent spotting is accomplished by offsetting the spotting position by a small distance. One of the disadvantages of this spotting technique is that the location of the samples (spots) on the slide does not correspond to the location of the samples (wells) in the source plate. Another disadvantage is that samples cannot be randomly accessed from the source plate and randomly printed on the slide. These disadvantages diminish the versatility and utility of such conventional microarraying technology.
Conventional pin transfer technology is also used in other applications such as high throughput screening (HTS). High throughput screening involves compound or reagent reformatting from a source plate to an assay plate. For example, test compounds, dissolved in DMSO are transferred from a 96 well plate to a 96, 384 or 1536 well microtiter plate. Typically, the desired transfer volume is higher than that for genomic arraying and is in the range from about 1 to 200 nanoliters (nL) or more. Undesirably, conventional pin transfer technology when utilized for compound reformatting can also suffer from some or all of the above disadvantages.
Microfluidic transfer of liquids can also be performed using an aspirate-dispense methodology. State-of-the-art aspirate-dispense methods and technologies are well documented in the art, for example, as disclosed in U.S. Pat. No. 5,741,554, incorporated herein by reference. These typically use pick-and-place (“suck-and-spit”) fluid handling systems, whereby a quantity of fluid is aspirated from a source and dispensed onto a target for testing or further processing. But to efficiently and accurately perform aspirate and dispense operations when dealing with microfluidic quantities, less than 1 microliter (μL), of fluid can be a very difficult task. The complexity of this task is further exacerbated when frequent transitions between aspirate and dispense functions are required. Many applications, such as DNA microarraying and HTS, can involve a large number of such transitions. In these and other applications it is desirable, and sometimes crucial, that the aspirate-dispense system operate efficiently, accurately and with minimal wastage of valuable reagents.
Therefore, there is a need for an improved technology and methodology that provides efficient, repeatable and accurate transfer of microfluidic quantities of fluid while reducing wastage of such fluids.